Field of the Invention
Embodiments of the invention relate to continuous-flow left ventricular assist devices (LVADs) and methods for their use.
Description of the Related Art
A left ventricular assist device or “LVAD” is a mechanical device that is typically placed in the chest or abdomen of a patient to assist with pumping blood in the patient's body. The LVAD is usually affixed so that it pumps blood from the left ventricle to the aorta of the patient. Normally an LVAD will include a cable that passes through the skin of a patient to allow control and assessment of the LVAD. The cable also allows connection to a controller, power pack, and, typically, a reserve power pack.
Long-term mechanical circulatory support is being used more frequently as bridge-to-transplantation and destination therapy for heart failure patients because of improved safety and reliability of left ventricular assist devices. In addition, mechanically unloading the ventricle reduces wall stress and myocardial oxygen consumption. This can lead to reverse modeling of the myocardium and recovery from heart failure in some instances. Recently, the use of continuous flow assist devices has become common due to their small size and valve-less design.
Unlike pulsatile LVADs, in which pump filling and ejection are determined in part by the patient's physiology and inlet cannula suction pressure is limited by atmospheric pressure, continuous flow LVADs produce a flow-dependent differential pressure as a function of pump speed as described by the characteristic pressure versus flow (H-Q) curve. If the speed is too slow, the patient may not receive an optimal amount of blood flow and their activities may still be limited by their heart failure. If the pump speed is too fast, the pump can empty the ventricle, pulling the ventricular wall towards the pump inlet and subsequently limiting flow. This phenomenon, referred to as a suction event, can cause myocardial damage and dangerous ventricular arrhythmias.
Infrequent but occasional aortic valve opening is often used as a guideline to set pump speed, indicating the ventricle has adequate residual volume at end-systole to prevent suction and is unloaded to some extent. However, the aortic valve opening is only measured in the clinic setting and is subject to changes in left ventricular (LV) contractility, heart rate, arterial pressure, and blood volume related to normal daily activities (e.g. sleep, exercise, positional changes, etc.). An automatic control algorithm is desirable to adjust pump speed in response to hemodynamic changes in order to provide sufficient support but reduce the risk of suction-induced arrhythmogenesis.
Of the many methods that have been devised to control continuous flow LVADs, most rely on an estimate of instantaneous pump flow based on the motor equation and power dissipation. However, accuracy is affected by the blood viscosity, model non-linearities, and noise in the power measurement. In addition, pump flow alone cannot determine the ventricular workload, and therefore, cannot be used to optimize pump speed to unload the ventricle. A control algorithm based on direct measurement of left ventricular volume and/or left ventricular pressure would be advantageous in preventing suction events and setting an optimal operating point that reflects ventricular loading. Once established, this control system can be used with existing pumps used clinically and allow adaptive flow control that can adjust with physiologic demand (i.e. changes in ventricular load). As LVAD patients leave the hospital and return to their daily activities, the control system will be able to adapt to the patient's circulatory needs and prevent adverse suction events.